Root Cause Analysis for TCH Drop for Ericsson System:Background:It has been observed that TCH Drop in Ericsson system is on higher side as compare to other vender system. Purpose of document is to find out the various reasons for call drop in Ericsson system along with solutions and features which helps for improvement in call drop.

Structure of the document:

1. TCH Drop Rate1.1 Define1.2 Measure1.2.1 STS counters for TCH drop1.3 Analyze1.3.1 Fish bone diagram for the root cause analysis for high TCH Drop Rate1.3.2 Power control parameters for MS1.3.3 Power control parameters for BS

2. Handover2.1 Define2.2 Measure2.2.1 STS counters and stats for handover performance2.3 Analyze2.3.1 Fish bone diagram for the root cause analysis for high handover failure rate2.3.2 E1 Algorithm for Locating (EVALTYPE=1)2.3.3 E3 algorithm for Locating (EVALTYPE=3)2.3.4 Urgency condition handover trigger parameters

3. New Features

1. TCH Drop Rate

1.1 DefineTCH drop (or a dropped call) could be broadly classified into 3 sub classes:

1. Degradation of the links (Uplink and Downlink): either degradation of Signal Strength which falls near or lower than the sensitivity of the base station (around to -110 dBm) or that of the mobile (around -104dBm) or degradation of quality of the links (Uplink and Downlink) often due to interference.

2. Excess TA (TA>63 or excess path imbalance due to high TA).3. Other Reasons.

BSC 1 TCH Drop Cause

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Drop Reason Low SS DL (%) Drop Reason Low SS UL (%)Drop Reason Low SS UL/DL (%) Drop Reason Bad Quality DL (%)Drop Reason Bad Quality UL (%) Drop Reason Bad Quality UL/DL (%)Drop Reason Suddenly Lost Connections (%) Drop Reason Excessive TA (%)Drop Reason Other (%)

Figure 1: an example of distribution of cause of TCH Drop in a BSC

1.2 Measure

Use the following available stats to measure nature of drop call rate in a cell / BSC

Drop Reason: Low Signal Strength DL %. Drop Reason: Low Signal Strength UL %. Drop Reason: Bad Quality DL %. Drop Reason: Bad Quality UL %.

Drop Reason: Sudden Lost Connections %. Drop Reason: Excess TA %. Drop Reason: Other %

Also the raw counters given in table 11 could be used for further analyse the cause of TCH drop. Always categorize the highest dropper (TNDROP) in a network by traffic and not just drop call rate, as an example take two cells in a network Cell A dropping at the rate of 20% with a net carried traffic of 30 calls in a day (say 0.025 Erlang) and Cell B dropping at the rate of 3.5% with a net carried traffic of 10,000 calls a day (say 42 Erlang). Though TCH Drop Rate for cell B is only 3.5% as compared to 20% TCH Drop Rate for cell A; Cell A drops 350 calls a day , where as cell B drops only 6 calls a day. Thus fixing the TCH drop issue on Cell A will show larger percentage of improvement at the BSC level as compared to Cell B.

1.2.1 STS counters for TCH drop

Counter Meaning CommentTF_TRAFFIC TF_TRAFFIC = (TFTRALACC/TFNSCAN) in Erlangs

T_DR_S T_DR_S =(TN_DROP/N_CALLS)*100 % TCH drop rate (DCR)N_CALLS I_CALLS+Inc(HO-AB-AW)-Outg(HO-AB-AW) Net sum of terminated calls in cell

I_CALLS = # of initiated calls in a cell (sum of four CASSALL for TCH or CMSESTAB for SD)Inc = sum of all incomming handovers to a cell from all its neighboursOutg = Sum of all outgoing handovers from a cell to all its neighboursHO is the number of successful handovers on TCH = HOVERSUCAW is number of successful assignment to worst cell =HOSUCWCLAB is number of succesful assignments to better cell =HOSUCBCL

TNDROP TN_DROP= TFNDROP+TFNDROPSUB+THNDROP+THNDROPSUB Total number of drops on TCHT_DR_S T_DR_S =(TN_DROP/N_CALLS)*100 % TCH drop rate (DCR)

TF_REL_C number of dropped TCH connection due to transcoder resource congestion (on TCH FR)at immediate assignmentTH_REL_C ;; on TCH HR in UL and OL bothTF_REL_C TF_REL_C=TFNRELCONG+TFNRELCONGSUBTH_REL_C TH_REL_C=THNRELCONG+THNRELCONGSUB

THNRELCONG ;; for HRTFNRELCONGSUB ;; for overlaid subcell FRTHNRELCONGSUB ;; for underlaid subcell in HR

TFNRELCONG# of released TCH signallign connections due to transcoder resource congestion during the immediate assignment on TCH. Transcoder Congestion

TFNDROP is incrimented at the same time.

Table 1: STS raw counters for TCH drop

1.3 Analyze1.3.1 Fish bone diagram for the root cause analysis for high TCH Drop Rate

TCH Drop Rate

Low Signal Strength DL Low Signal Strength UL

Bad Quality DL Bad Quality ULHigh TA /RF Spillage /Path Imbalence

External Interference

Figure 2: Fish bone diagram for the root cause analysis for high TCH Drop Rate

TCH Drop Rate

Hardware Faults Drops due to Other Reason

Power Control Sudden Lost ConnectionHandover Failures

HCSCLS

Assignment to another cell

Figure 3: Fish bone diagram for the root cause analysis for high TCH Drop Rate

Low Signal Strength UL: UL is the weakest of the both DL and UL on an average in a network; the uplink is seen to be 3 dB weaker than DL. Hence the contribution of weak signal on the UL to TCH drop call rate in a network will usually be far greater than that by weak signal strength on the DL (check figure 16, for BSC1 DL weak signal strength contributes to 1.25% of the net cause of TCH Drop Rate where as weak signal strength on the UL contributes to 36.56%); this means that the UL in any network will be more prone to

interference (that is a lower C/I as I staying constant on both DL and UL , and since C is lower in case of UL , naturally C/I for UL will be lower for UL).

Optimisation of UL can either be done using, RF coverage optimization, Hardware fault checks or parameter optimisation.

1. RF Coverage Optimization for UL: Antenna load heights , tilts , azimuth , weak coverage cluster optimization, addition of more sites , using Micro cells to cover hotspots, use of TMA to give additional gain on the UL etc.

2. Hardware fault checks: Faulty Antenna, High VSWR due to incorrect termination of RF cable , RF Jumper and antenna feed mechanisms, damaged connectors , damaged cables ,water in external RF feed cables faulty TRX1 etc.

3. Parameter Optimization: In idle mode ACCMIN & CRO could be optimised to make sure that the MS makes correct cell reselection, also make use of the feature Assignment to Another Cell (assignment to better cell part of this feature) for these cases of incorrect cell reselection in idle mode. MSTXPWR defines the maximum permitted transmit power from the MS, make sure its set at 43 (1.9 Watts) for 900 band and 30 (1 Watt) for 1800 band. Another set of cell parameters that influences UL signal strength performance is UL power control settings; make sure SSDESUL is not set too low (-95 dBm or weaker than that for a cell that drops excessively on the UL) , LCOMPUL and QCOMPUL settings are not too high (increasing the value of these two parameters leads to aggressive power control on UL) , filter lengths for signal strength SSLENUL and uplink quality QLENUL are set between 3 to 5 seconds for faster power revision commands form the BSS especially for mobiles in the weak coverage areas of the cell , where signal strength fading rate can be very rapid. Also for cells with high percentage of drops due to weak uplink levels make sure that the desired quality for the UL is set very sensitive, use the parameter QDESUL to control this aspect (keep it a 0 instead of default 20). Power control is covered in more detail later in this document.

Low Signal Strength on DL: Excess reasons for drops on weak DL are similar to that of drops on weak UL level. Drops due to weak levels on DL for any cell should always be lower than that due to weak levels on the uplink , if its the other way round either its because of hardware issues similar to that discussed above or due to incorrect settings of BSTXPWR. For cells with wide footprint and high density of traffic from far-off zones (high TA & high distribution of traffic at far-off zones seen on MRR recordings) , excess drops are seen due to path-imbalance ; idle mode resection and eventual channel request are made based on DL measurements , which means in these kind of cases an MS might camp on a cell with reliable DL but unreliable UL which it only realises either during RACH, AGCH and SDCCH signalling process or when the MS lands on the TCH , where it drops due to weak and unreliable UL (rather than DL related issues). Thats the reason why power control optimization of UL or use of TMA usually brings in much better performance improvements in a network than similar work done on DL.

Bad Quality UL: The reasons for excess drop due to bad UL quality could be classified as Interference related (Co Channel interference , Adjacent Channel interference or External Interference) , coverage issues, handover issues or incorrect cell parameter settings.

Once again if correct power control settings are used at the BSC/Cell level with a good frequency plan and use of synthesizer frequency hopping can bring down excess drops on UL due to bad quality.

1. Interference: Check for co-channel, adjacent channel interference and make the relevant frequency changes.

2. Coverage: discussed in the last section.

1 Excess VSWR is a common observed problem in networks with weak maintenance plan. High reflected power back to the radios often causes damage of sensitive Low Noise Amplifiers in the Rx Path causing excess UL related drops.

3. Parameters: Correct settings for idle mode reselection parameters (ACCMIN and CRO) Power control settings (covered later) , use of DTX2, MSTXPWR , QLIMUL3

4. Handovers: missing neighbours, incorrect settings for locating algorithm etc can lead to drag and drop scenarios.

External Interference: External to GSM system, but operating frequency band, or harmonics falling within GSM band. Use a spectrum analyser in field to measure and identify the source.

High TA / RF Spillage and Path imbalance: Covered under the section of Low Signal Strength UL.

Drops due to Other Reason: This is often a difficult one to crack , excess TCH drop due to other reason could be due to the following reasons:

1. Transcoder synchronization fault, the counter TRASYNCCOUNT gets incremented when TRA sync fault is reported by the BTS on any of the timeslots within the TG4.

Relationship between TCH Drop Rate andTranscoder Synchronization Faults

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Figure 4: Relationship between increase of TCH Drop Rate and the increase of transcoder synchronization fault (TRASYNCCOUNT from STS

MOTG object type)2. Faulty Transcoder5.3. TRAB (vododers) congestion; use the counter TFNRELCONG & THNRELCONG

counter to check this.

2 Use of Discontinuous Transmission on the UL ( DTXU=0 , MS may use discontinuous transmission ) brings down interference on UL3 QLIMUL determines the urgency condition for UL quality based handover trigger value, default setting of 55 (i.e. Rxqual of 5.5 for 4 SACCH period leading to urgency based handover) ; if quality drops on UL is seen to be very high , reducing QLIMUL is seen to bring down TCH Drop Rate.4 TG or Transceiver Group is a MO (managed object) within the RBS5 If a transcoder card is seen to be faulty use the transcoders in a pool feature to minimize localised high TCH drop to a BSC by distributing the issue across cells from more than one BSC till the faulty transcoder card is replaced.

4. Blocked A-interface (RALT) and ABIS (RBLT) devices causing A and ABIS interface problems.

5. C7 link problems (link unavailability, high BER).6. LAPD problems (link failures, high BER).7. If LAPD problems (excess T2006 timer expiry) are not due to transmission related

issues (congestion, availability or high BER due to interference), give a reset to DXUs on TG that is showing LAPD problems).

8. Intermittent digital path (DIP) quality problems on transmission networks caused by interference on the transmission networks.

9. Caused by usage of features like BTS/MS power control (high discrepancy in power control settings in a BSC causing excessive CP load) or Adaptive Configuration of Logical Channels.

10. Very high load of Location Updating Requests within the BSC.11. LAPD Concentration factor7 set to too high value resulting in LAPD congestion.

TCH drop due to Sudden Lost Connection: This can be due to almost all the reasons from TCH drops due to other reason plus RF coverage issues. An example of TCH drop due to sudden lost connection could be a subscriber entering a lift while in conversation; apart from these reasons check also for loose feeder cable/jumper cable connections to the antenna, connectors and to the RBS.

Hardware faults: Faulty antenna, RF cabling issues, VSWR, BTS faults (check alarms). Handover: Missing neighbours, swapped sectors, locating algorithm needing corrections,

high number of co-BSIC/BCCH combinations in a cell, RF coverage issues etc. This topic is covered in detail in the next section.

CLS: Aggressive use of CLS, especially for cells with weak or interfered neighbours can cause high TCH Drop Rate (check per neighbour handover success rate and only then optimise CLS settings).

HCS: In a dual band network, often to make the 1800 band carry more traffic HCS parameters are set too sensitive leading to drop on the higher bands. Very low multiband parameters like LAYETHR and HSCBANDTHR often leads to high TCH drop rate on the 1800 layer.

6 T200 timer comes into picture mainly during Call Setup, Assignment and handover (signalling part) and it monitors the decodability of the signalling information (from SDCCH, FACCH and SACCH) on the signalling frames. If the information in the frames are not decodable T200 timer is decremented , and if it gets continuously decremented by a figure specifies by N200 (23 for SDDCH or approximately 220 ms , 34 for FACCH used for handover or approximately 115ms or 5 for SACCH or 900 ms), BTS will send a Layer 3 message Error indication- abnormal release reason unspecified (cause value =1) to both MS and BSC along with T200 expiry , both these messages could be read from the Layer 3 messages on the MS ( TEMS ) or from CTR measurements at the BTS. BSC in its turn will drop this call/setup attempt and increment the counter CNDROP and treat it as a drop due to other reason. Often interference , or incorrect settings of ACCMIN is the cause of excess T200 expiry in a cell ; but if all the three sectors shows excess T200 expiry (T200 expiry common to the site) then the cause often is transmission related.7 LAPD Concentration factor: The main components of an RBS is the TRUs (radio that helps setup and sustain a call from MS), which is connected to a DXU which acts as the interface/switching unit between the E1-transmission connection in and out of the RBS to the BSC (this E1 connection is also referred to as DIP or a PCM). 1 E1 connection to an RBS (terminated on the DXU of the RBS) has 32 PCM timeslots in all with each timeslot having a capacity of carrying speech information from 4 speech timeslots from a TRU, that is each timeslot on the E1 has a 64kBps bandwidth (and hence 16*4 air speech timeslots mapped onto one PCM timeslot). Now each TRU w.r.t. to this E1 link to the DXU within the RBS needs a whole 64 Kbps TS for FACCH , BCCH , SCH , CCCH(paging), SDCCH and SACCH ; this is because only TCH information gets send at sub rate of 16 Kbps where as signalling is not sub rated and doesnt pass through the transcoder). Now one whole PCM timeslot needed for signalling per TRU can bring down the available bandwidth for TCH traffic on the E1 connection between RBS and BSC. Thats the reason why a component called CON (concentrator) is used in the RBS. CON is connected direct to the DXU and all the signalling timeslots from TRU gets routed through CON to the DXU. CON defines the priority of the signalling information and makes sharing of a PCM timeslot between TRUs possible; this is done using the cell parameter CONFACT (default value =1, range =1 to 4). Setting it at high values , say =4 , CONFACT=4 means FOUR TRUs from the RBS will share ONE PCM timeslot on the E1 instead of using 4 PCM timeslots. Such settings can increase the number of TRUs that could be equipped on one E1 between the BSC and the RBS but at the same time will put immense load on the signalling bandwidth (bringing down signalling performance, increasing TCH drops due to other reason, lower paging success rate etc) especially in the case of cells that carry a lot of traffic.

Assignment to Worse Cell: This feature can increase the TCH drop Rate at the expense of brining down congestion in cases where the neighbours are weak or interfered. Check per neighbour handover success rate before implementing this feature.

MS and BS Power Control: Power control is usually seen to have huge impact on both SDCCH and TCH drops especially on the UL, improving both call setup success rate and drop call rates both on SDCCH and TCH. Excess transited power, on the downlink by the BS as well on the uplink by the MS, often causes proportional excess interference on the links.

Adverse impact of excess transmit power by calls originated from close in fields on calls which are originated from far off fields is usually acute; reasoning here being, received signal strength at the MS for far off traffic is weak due to excess fading from longer propagation distance and hence prone to more damage from interference.Controlling the high percentage of excess transmit power in the close in fields can bring down the interference levels on uplink (and downlink).

1.3.2 Power control parameters for MS

Parameters Default Recommended Range Unit Comment

SACCH periods time interval(min) between two power orderREGINTUL 5 1 1 to 30

dtqudesired UL quality ; if aggressive power control of UL SS is used , keep it at 0 (sensitive on

INIDES -70 -80 neg100 to neg 47 dBmSDDCH : ininitial desired signal strength for SDCCH (keeping it on -70 can cause drop on

QDESUL 20 0 ,10 or 20 0 to 70

SACCH periodsfilter length for SDDCH power control , for faster power adjustments on SD keep it low

QLENUL 8 3 1 to 20 SACCH periodsLength of quality filter on UL , keep it low to react queckly(increase power) degradation of rxQual

INILEN 2 2 0, 2 to 5

%UL quality compensation factor,high values of QCOMPUL: faster response to degradation of

SSLENUL 5 3 3 to 15 SACCH periodsLength of signal strength filter , for cells with fast moving traffic keep it low

QCOMPUL 30 75 0 to 60

dBmdesired signal on the UL if rest all criterias(path balance and interference ) are met.

LCOMPUL 70 6 0 t0 100 %UL path loss compensation factor, increasing LCOMP will increase aggressiveness of UL Pcon

SSDESUL -85 -95 neg110 to -47

Table 2: Power control parameters for UL

SSDESUL: keeping it low will make sure that when the UL path loss (controlled by LCOMPUL) is low (that is the MS is very close to the site where from it need not transmit at very high power to reach comfortable to BS) and the UL quality is very good (QDESUL=0, RxQual UL is 0; QCOMPUL=75), the MS will be powered down to very low transmit power thus decreasing the overall noise floor for the BSC. This strategy of aggressive power reduction on MS and at the same time making the uplink extremely sensitive to high path loss and interference (power up immediately on interference or higher path loss) is seen to bring down TCH drop and SDCCH drop by huge factor.

Also at the same time keeping the filter lengths low ensures that power change commands are send to MS at a faster rate to keep the link under high interference due to low levels or high path loss.

UPDWNRATIO: This parameter controls the ratio between up and down power regulation speed. Default value for UPDWNRATIO is 200. When using the above strategy for UL power control keeps this parameter at 300.

1.3.3 Power control parameters for BS

Parameters Default Recommended Range Unit CommentSSDESDL -70 -90 neg 110 to -47 dBmQDESDL 20 30 0 TO 70 dtquLCOMPDL 70 5 0 to 100 %QCOMPDL 30 55 0 to 100 %REGINTDL 5 1 1 to 10 SACCH PeriodsSSLENDL 5 3 3 to 15 SACCH PeriodsQLENDL 8 3 1to 20 SACCH PeriodsSDCCHREG OFF ON ON,OFF Enables power control on SDCCH

Table 3: Power control parameters for BS

2. Handover

2.1 Define

Handover in Ericsson BSS system is controlled by what is called as the locating algorithm in the BSC. Locating algorithm operates on the basis of Measurement Reports (MR)8 sent in by the MS on SACCH.

The inputs that the BSC uses for making a handover decision, from the received MRs from the MS is the DL signal strength, DL quality, and the signal strength of the six best reported neighbours. From the serving BTS, for the same MS the BSC will use UL signal strength, UL quality and TA.

There are two Location algorithms E1 and signal strength based E3. E1 algorithm uses path loss measurements and received signal strength measurements to come to a handover decision. For both E1 and E3 the BSC goes through the same sets of steps before coming out with a need of a handover decision; these steps are MR filtering, urgency condition detection (quality urgency or TA urgency), radio network function evaluations and basic ranking of the serving and reported six best neighbours.

2.2 Measure2.2.1 STS counters and stats for handover performance

8 MR called as Measurement Reports are sent up by the MS in/on every SACCH interval (480 mS) when in Active mode. The BSC primarily uses the information in MR sent up to it on SACCH to make power control and handover decisions. If the BSS cannot decode MRs for the time period set by radio link timeout counter (usually set for 16 SACCH periods, or 8 seconds) will drop the call. A Measurement Report usually consists of the following information: BA used, DTX used, RxLev Full and Sub for the serving cell and 6 best neighbours, RxQual Full and Sub of the serving cell, TA of the serving cell and decoded BCCH-BSIC of the six best neighbours.

Counter Meaning CommentHOVERCNT Number of handover commands sent to the MSHOVERSUC Number of successful handovers to the neighbouring cellHODUPFT Number of successful handovers back to old cel within 10 secondsHOTOKCL Handover attempt made to better K-cell ; corresponding L cell is called HOTOLCL for Ericson 1 algorithmHOTOHCS Handover attempt due to HCSHODWNQA

HOEXCTA Number of handover attempts due to excessive timign advanceHOASBCL Number of assinment attempts to better cellHOASWCL Number of assinment attempts to worse cellHOSUCBCL Number of successful assignment attempt to better cellHOSUCWCL Number of succesful assignment to worse cellHOATTLSS Number of handover attempts when serving cell is a low signal strength cellHOATTHSS Number of handover attempts EVEN when serving cell is a high signal strength cellHOATTHR Number of handover attempts at high handover rateHOSUCHR Number of successful handovers at high handover rateH_LOST H_LOST = [(HOVERCNT-HOVERSUC-HORTTOCH) / HOVERCNT]*100 % HO lost ; Ho succ rate

Number of hadover attempts due to bad downlink quality ; HO counter for bad UL quality is HOUPLQA

Table 4: Counters and stats available for Handover

Apart from these counters STS also keeps per neighbour handover success rate for all the available neighbours to a cell. Per neighbour handover success rate gives us immense amount of information on the performance between two neighbours, strength/ weakness of RF overlap between two cells, interference issues between the borders of two cells etc and could be used to identify weak neighbours for RF, handover optimisation to improve upon TCH drop caused by handover failures.

2.3 Analyze

2.3.1 Fish bone diagram for the root cause analysis for high handover failure rate

Poor HO Succ Rate

Co -BCCH /BSIC Missing Neighbours

TCH Congestion on target Swapped SectorsWeak Cell Boundry

TCH interference

Too many nbrs

Incorrect Parameters

Figure 5: Fish bone diagram for the root cause analysis for high handover failure rate

Missing Neighbours: make use of planning tool, NOX9 and drive tests to identify and add missing neighbours.

Co-BCCH/BSIC: Often a major contributor to handover failures in a network. MS when in active mode reports back the six best detected neighbours(MS in active mode measures only those BCCH supplied to it on system information from the current serving cell , this list supplied to the MS in active mode by the serving cells is the neighbour list of the serving cell or also called as the SACCH list of the serving cell) this six best sorting done by MS strictly is based on the reported neighbours DL signal strength level , and gets reported back to the BSC in the MR sent up on SACCH. This information on the measured neighbours consists of RxLevel of the neighbours and reported BCCH and BSIC of the neighbour. If within the coverage boundary of a cell , there exists another cell (spillage) with similar BCCH as the reported neighbour , then it can often cause incorrect decoding of the relevant neighbours BSIC (or no decoding, depending upon the interference level by the spiller). Such a situation can lead to a handover failure leading to TCH drop. Do an audit of the co BCCH/BSIC allocation within the BSC and surrounding BSC and change co BSIC for cells with same BCCH.

TCH Congestion at Target: Handover signalling between two cells is performed over FACCH. FACCH operates by stealing a bit from the TCH channel that is serving the MS currently (this means the whole signalling process of handover happens on TCH itself). When the BSC initiates a handover process over FACCH of the serving cell direct to TCH channel of the best reported neighbour (once again FACH of the neighbour) and if the neighbour is congested (TCH congestion at target) it will lead to a handover failure at the source cell due to a cause target cell congested. If such signalling failure occurs during a handover BSC will apply penalty of PSSHF (default setting 63, which is 63 dB and a range of 0 to 63 dB) for a time period of PTIMHF (default setting of 5 sec, range of 0 to 600 second), this penalty is applied to a handover for the mobile under consideration and not for other mobiles in the same source cell or other cells trying for handover to the neighbour under dispute. After applying this penalty BSC will do the process of locating again and if the same congested neighbour is reported as the best neighbour, after the period set by PTIMF the MS will retry for a handover. Excess congestion at a target cell with a lot of inward mobility can lead to high handover failure rate at the source cells and often TCH drop due to drag and drop scenario.

Swapped Sectors: Antenna implementation fault (with un-swapped neighbour definition). BCCH / TCH interference: At the cell boundaries causing either difficulty in decoding corrupt

MRs or signalling failures due to high interference on TCH (on FACCH). Weak Cell Boundary: Poor RF coverage across the cell boundary will make decoding of

SACCH and FACCH difficult leading to handover failures or TCH drop. Too Many Neighbours: In a GSM system a cell can have up to 32 defined neighbours. But

for a BSC a combination of high density of traffic, high percentage of mobile traffic needing handovers, with majority of the cells having very high count of neighbours means too much processing ; which can lead to either too high CP load or very high amount of signalling on the transmission networks (especially on the ABIS) leading to link congestions. For such BSCs brining down the average neighbour count per cell can marginally improve handover performance across the BSC.

Incorrect Handover Parameters: Optimization of handover parameters (for both E1 and E3 algorithm) can lead to improvement of HO success rate in a network.

2.3.2 E1 Algorithm for Locating (EVALTYPE=1)

9 NOX: Neighbour Cell List Optimization Expert is an OSS feature from Ericsson which can detect missing neighbours by rotating test neighbours in a cell across the legal frequency band, and measure the signal strength reported from MS for these test neighbours (BCCH/BSIC combinations). If any of the reported BCCH/BSIC shows very good levels (thresholds could be pre set), NOX can either add it as a neighbour or else create a missing neighbour report.

Parameters Default Recommended Value Unit Comment

dB

Hysteris appiled to LOFFSET , to prevent ping pong handover.

LOFFSET 0 0neg 63 to +63

dBLink balance criteria , positive value will make the sorce cell more attractive to retain the call (strech the call into target),delay the

LHYST 3 3 0 to 63 dB

Handover Margin , absed on signal strength criteria ; neighbour needs to stronger that server by this factor to be considered for handover calcul.

TROFFSET 0 0neg 63 to +63

dBfactor by which a cell boundry is displaced away from the target (for handover) ; delays handover (ping pong red.)

TRHYST 2 2 0 to 63 dBsimilar to KHYST , but applied over Link balance criteria of E1 and it reduces unnecessary handovers

KOFFSET 0 0neg 63 to +63

dBmsufficient Receaved level on the UL from a neighbour to be considered as a candidate hor handover evaluation

KHYST 3 3 0 to 63 dBUsed to decrease(applied penalty) ranking value for a neighbouring cell (reduce ping pong handovers)

BSRXSUFF neg150 to 0

MSRXSUFF neg150 to 0 dBmsufficient Receaved level on the DL from a neighbour to be considered as a candidate hor handover evaluation

Table 5: E1 Locating algorith parameters

In E1 algorithm, the BSC uses the above mentioned parameters to rank handover candidates into two lists; the L-List (link balance list) and K-list (signal strength list). This ranking is done as per figure 21

Figure 6: Basic Candidate List by Locating Algorithm

This kind of listing by E1 algorithm will ensure that a handover decision takes place only when the mobile is far away from the serving cell (delay the handover till the link budget becomes an issue or an emergency handover trigger either in terms of bad quality (QLIMULor QLIMDL) or TA (TALIM) has occurred. 2.3.3 E3 algorithm for Locating (EVALTYPE=3)

Parameters Default Value Recommended Value Value Range Unit Comment

dBHandover hysteris (neighbour should be strong by this factor) applied to the serving cell if it is tagged as "high

OFFSET 0 0 neg 63 to 63 dBhandover margin (the neighbour should be stronger than the server by OFFSET+hysterisis to be considered for a

HIHYST 5 3 0 to 63

dBmThe thhreshod below which the the serving cell gets tagged as "low signal strength cell" .

LOHYST 3 3 0 to 63 dBHandover hysteris (neighbour should be strong by this factor) applied to the serving cell if it is tagged as "low signal

HYSTSEP -90 neg 150 to 0

Table 6: E3 algorithm for locating Ericsson Algorithm 3 (algorithm type is determined by the BSC parameter EVALTYPE) is easy to implement and optimize.

Urgency Condition in handover: Urgency conditions are special conditions where a handover is a must even if the locating algorithm has not found a need to perform a handover to the best neighbour located in the candidate list. Bad Quality issues and excess TA issues trigger an urgency condition handover. Parameters controlling urgency condition handovers are listed in table 17.

2.3.4 Urgency condition handover trigger parameters

L-list

K-list

Lowest L -rank

Highest L -rank

Best cell

Worst cell

Highest K -rank

Lowest K -rank

L-list

K-list

Lowest L -rank

Highest L -rank

Best cell

Worst cell

Highest K -rank

Lowest K -rank

Parameters Default Recommended Range Unit Comment

TALIM snormal range 62 62 0 to 63 bit periodextended range 62 0 to 219 bit period

s time period for which PSSTA is applied

EXTPEN OFF OFF ON,OFF alllows or dissallowes urgency condition inter-BSC handovers

PTIMTA 30 30 0 to 600

dB time period for which PSSBQ is applied

PSSTA 63 63 0 to 63 dBpenalty given to the cell abandoned on high TA (for that particular connection , that MS)

urgency condition due to high TA

PTIMBQ 15 15 0 to 600

dtqucell to cell symetrical relation, the negative offset in level acceptable to a urgency handover(to a worse neighbour) on

PSSBQ 10 7 0 to 63 dBpenalty given to the cell abandoned on bad quality (for that particular connection , that MS)

BQOFFSET 3 3 0 to 63

dtquthreshold for UL quality emergency (triggers an handover if UL Qual reaches RxQual of 5.5 or worse for quality filter length

QLIMDL 55 55 0 to 100 dtquthreshold for DL quality emergency (triggers an handover if DL Qual reaches RxQual of 5.5 or worse for quality filter length

QLIMUL 55 55 0 to 100

Table 7: Urgency condition handover trigger parameters

3 Enhanced Handover

1 Introduction

This document describes how to improve the robustness of the system by enhancing the control signalling performance of the FACCH and SACCH. FACCH is mainly used for Handover and Assignment signalling; lost handovers or assignments are mainly due to poor FACCH performance. SACCH defines the Radio link time out counter which generally defines a dropped call.

The features covered in this document are Enhanced Handover Success Rate and Enhanced AMR Coverage. The features are implemented according to the 3GPP standard, see Reference [4].

1.1 Background

Investigations and field trials have shown that the speech frames are more robust than the associated control signalling and the control signalling may therefor terminate calls to early. By improving the control signalling, drop rates in networks with high frequency utilization or low signal strengths may be reduced even further. The features Enhanced Handover Success Rate and Enhanced AMR Coverage have been developed for more robust FACCH and SACCH signalling.

2 Capabilities

The downlink FACCH performance is improved by using the feature Enhanced Handover Success Rate. The feature increases the probability of decoding the Layer 3 FACCH messages for MSs in bad radio conditions and low signal strength. The feature consists of two parts:

Repeated Downlink FACCH Re-establishment of Layer 2

Repeated Downlink FACCH sends the Layer 3 messages in pairs. The MS then have two opportunities to decode the Layer 3 message instead of just one, which may improve the Handover Success Rate and the Assignment Success Rate. This applies for all MSs, but the benefit is even higher if the MS is capable of soft combining, as specified in Release 6 of 3GPP.

Re-establishment of Layer 2 is triggered if the Layer 2 signalling is lost in the originating cell and applies to all MS.

The feature Enhanced AMR Coverage improves the SACCH performance on either link independently. To increase the probability to decode SACCH messages the same message is sent twice when the conditions deteriorates due to low signal strength or interference. The feature requires 3GPP Release 6 MSs capable of soft combining. The function will be enabled for MSs that are in bad conditions and have reported apability of the feature. When the conditions improve, the functionality will be disabled automatically.

The functionalities mentioned above are expected to work well in all networks. The concerns that could occur are:

- Repeated Downlink FACCH steals more speech frames than normal FACCH transmissions.

However the five first transmissions of a message are always without the repetition enabled, which covers the normal performing FACCH messages. After the five first attempts the risk for a drop call is imminent when the feature is not activated.

- Enhanced AMR Coverage reduces the number of Measurements Results to half

The number of messages is still the same, but every second is a copy of the previous one. This means that the information is not updated as often for locating and power control when in repeated SACCH mode. However, if the SACCH frames are lost, a dropped call is imminent, making this measure well motivated. It is important to keep in mind that the MS only goes into repeated SACCH mode when really needed.

2.1 Benefits

These features will reduce the drop call rate.

3 Technical Description

3.1 Enhanced Handover Success Rate

The feature Enhanced Handover Success Rate is activated per BSC. The feature consists of two functional parts:

Repeated Downlink FACCH Re-establishment of Layer 2

Repeated Downlink FACCH and Re-establishment of Layer 2 can be activated independently. To assist Enhanced Handover Success Rate an improved FACCH decoder in the BTS was introduced in 06B, this decoder contributes to enhancing the feature.

The feature improves all networks to some extent, but shows the largest improvement on networks with high spectrum utilization or low signal strengths. MSs capable of soft combining (i.e. 3GPP Release 6 MSs) benefit the most from this feature. There is no impact on legacy MSs. There are no drawbacks with using this feature regardless of the network configuration.

If the MS faces a bad radio environment and a Layer 3 downlink FACCH message (Reference [5]) is sent to the MS, the MS may have difficulties to decode the message. This feature is used to increase the probability that the MS successfully decodes the message by sending the repeated messages in pairs.

If the MS still has not been able to decode the message despite the repetitions and the message is a Handover Command or Assignment Command, the BSS tries to re-establish the Layer 2 signalling in the originating cell in order to maintain the call.

For more information about the handover process, see Reference [3].

3.1.1 Repeated downlink FACCH

Usually when sending a Layer 3 downlink FACCH message the message is repeated until it is decoded. When Repeated Downlink FACCH is used the repetitions of a Layer 3 downlink FACCH message are sent in pairs with a time distance of 40ms within the pair. When sending the repetitions in pairs the probability of successful decoding is increased. 3GPP Release 6 MSs, capable of soft combining, can combine two messages which increases the probability of correct decoding. Figure 1 shows how the messages are sent when Repeated Downlink FACCH is activated and when it is not.

Figure 1 Definition of T200 without (A) and with repeated FACCH (B).

In a bad radio environment during the handover process, the mobile station may have difficulties decoding the layer 3 downlink FACCH message successfully. Usually the message is sent until a response is received or until the message has been repeated the maximum number of times, see the example below. When the Repeated Downlink FACCH functionality is used the repetition of the message is started. If the message is not decoded during the five first messages, the repetitions are sent in pairs. This keeps the signalling load on the FACCH channel at a low level, since most messages are decoded within the first five repetitions for MSs outside a

3.1.1.1 Handover Command or Assignment Command not segmented

When the Handover Command or Assignment Command is not segmented, the MS will try to reach the target cell (or target channel in case of Assignment Command) as soon as the Handover Command or Assignment Command message is decoded by the MS.

3.1.1.2 Handover Command or Assignment Command segmented

When the Handover Command or Assignment Command message is segmented, the MS will not try to reach the target cell until the last Handover Command or Assignment Command segment is decoded by the MS. The MS has to send ACK/NACK on each decoded segment of the Handover or Assignment Command message.

3.1.2 Re-establishment of Layer 2

If a Handover Command or Assignment Command is sent to the MS and there is no response from the MS an Error Indication is received in the BSC. Usually the call is disconnected when the Error Indication is received, but when Re-establishment of Layer 2 is activated the system tries to re-establish the call on the originating channel again, also see Figure 2.

Figure 2 Re-establishment of Layer 2 signalling. If the MS fails to decode the Handover Command or Assignment Command, the network tries to re-establish the LAPDm signalling on the originating channel.

3.1.3 Value Ranges and Defaults Values

The timers introduced by the feature Enhanced Handover Success Rate are T_ESTGUARD, T_REESTABLISH and T_REESTGUARD, for default values, recommended values and value ranges see Reference [1].

3.2 Enhanced AMR Coverage

The feature Enhanced AMR Coverage is an optional feature activated per BSC that increases the probability of successful decoding of SACCH messages.

Enhanced AMR Coverage increases the probability of successful decoding of a SACCH block by combining two transmissions of the same block. This is done by repeated SACCH mode and 3GPP Release 6 MSs capable of soft combining. Repeated SACCH mode is adaptively switched on when necessary.

Usually a new SACCH block is sent every 480 ms. When in repeated SACCH mode the same SACCH block is sent twice within 960 ms, i.e. the time between two SACCH blocks is still 480 ms but every second SACCH block is replaced by a copy of the previous block. The amount of information transmitted when in repeated SACCH mode is thus half of when normal SACCH transmissions are used. The repeated SACCH mode could be seen as having a BLER of 50% on normal SACCH transmissions. In order to use it only when it is really necessary the repeated SACCH mode is adaptively switched on when the SACCH performance is poor. The repeated SACCH mode is switched off when the SACCH performance is recovered.

In order to control the performance, internal leaky bucket counters are used per call; one for the uplink and one for the downlink. The counters are incremented by two when a SACCH block is unsuccessfully decoded and decremented by one for a successfully decoded SACCH block. When these counters are greater than or equal to a threshold value (SACCHDLTHR for the downlink and SACCHULTHR for the uplink) the call goes into repeated SACCH mode. For more information about the thresholds see Table 2. The repeated SACCH mode is switched off when the counter value has returned to zero. If the counter value is equal or greater than the threshold, the counter value is not increased further if another SACCH Block is unsuccessfully decoded. If the counter value is equal to zero it is not decreased further when a SACCH Block is correctly decoded.

Figure 3 Measurement Reports sent on the uplink SACCH when in normal SACCH mode and in repeated SACCH mode.

Enhanced AMR Coverage improves decoding of Measurement Reports on the uplink and System Information on the downlink. Figure 3 illustrates how the uplink SACCH messages are sent when in repeated SACCH mode compared to normal SACCH trabsmissions. This is valid for System Information messages on the downlink as well.

3.2.1 BSC commands

RFEAI Initiate Enhanced AMR Coverage in the BSC.

RFEAE Deactivate Enhanced AMR Coverage in the BSC.

RFEAC

Change Enhanced AMR Coverage threshold values for UL and DL respectively.

o SACCHULTHR to set the uplink threshold.

o SACCHDLTHR to set the downlink threshold.

RFEAP Print the status of Enhanced AMR Coverage.

For more information about the BSC commands, see Reference [2].

3.2.2 BSC parameters

The threshold values introduced by Enhanced AMR Coverage are SACCHDLTHR and SACCHULTHR, for default values, recommended values and value ranges see Table 2.

Table 2 Threshold values

Parameter Name Default Value

Recommended Value

Value Range Unit

SACCHDLTHR 8 8 1 to 63 -

SACCHULTHR 8 8 1 to 63 -

3.3 Related Statistics

3.3.1 KPI impact

3.3.1.1 Enhanced Handover Success Rate

When using Enhanced Handover Success Rate the robustness of the system is expected to improve by enhancing the FACCH signalling performance.

The affected KPI:s are:

Handover Success Rate Handover Lost Rate Handover Reversion TCH Drop Rate

3.3.1.2 Enhanced AMR Coverage

When using Enhanced AMR Coverage the robustness of the system is expected to improve by enhancing the SACCH signalling performance.

The affected KPI is:

TCH Drop Rate

3.3.2 Impact on Legacy Counters

3.3.2.1 Enhanced Handover Success Rate

When introducing the feature Enhanced Handover Success Rate the HORTTOCH and HOINBOCH counters were updated.

HORTTOCH is also stepped at unsuccessful inter cell handover attempts when the network succeeds to re-establish the call on the originating cell.

HOINBOCH is also stepped at unsuccessful intra cell handover attempts when the network succeeds to re-establish the call on the originating channel.

3.3.3 Statistics for Performance Management

3.3.3.1 Enhanced Handover Success Rate

The value of the Handover Lost KPI is expected to improve since the HORTTOCH counter is changed as mentioned in Section 3.3.2. However the formula for Handover Lost remains the same.

3.3.3.2 Enhanced AMR Coverage

There are four new STS counters introduced to monitor the fraction of time in repeated SACCH mode:

EASULACTMREP stepped for each Measurement Report that is received while in repeated SACCH mode on the uplink.

EASULCAPMREP stepped for each Measurement Report that is received from an MS capable of repeated SACCH, while the feature Enhanced AMR Coverage is activated in the BSC.

EASDLACTSBL stepped for each DL SACCH block received by the MS while in repeated SACCH mode on the downlink.

EASDLCAPSBL stepped for each DL SACCH block received by an MS capable of repeated SACCH, while the feature Enhanced AMR Coverage is activated in the BSC.

Use EASULACTMREP/EASULCAPMREP to calculate the fraction of time in repeated SACCH mode on the uplink.

Use EASDLACTSBL/EASDLCAPBL to calculate the fraction of time in repeated SACCH mode on the downlink.

3.4 Main changes in Ericsson GSM system 07B/ BSS 07B

The feature Enhanced AMR Coverage and an enhancement of Enhanced Handover Success Rate also called Extended Repeated FACCH was introduced. In 06B only Handover and Assignment Commands were included in Repeated downlink FACCH, but in 07B all Layer 3 downlink FACCH messages are included.

4 Engineering Guidelines

4.1 Enhanced Handover Success Rate

4.1.1 MSC timers

In order to avoid that the MSC disconnects the call while Enhanced Handover Success Rate is used, there are three Ericsson MSC timers (TIMHNDCMDINTRAM, HNDTCMDBASNON and THNDCMDBASICM) that need to be set in order not to time out at inter BSC handovers. The MSC timers must be bigger than the sum T8 (BSS(BSSMAP))+T_ESTGUARD+T_REESTABLISH+T_REESTGUARD in order not to time out while Enhanced Handover Success Rate is used. For recommended values, value ranges and default values see Table 1.

4.2 Enhanced AMR Coverage

In order to monitor the performance of Repeated SACCH capable MSs it is possible to use the filtering function in R-PMO.

When a call is set up the system needs to know if the MS is capable of using the feature as soon as possible and therefor it is recommended to use early classmark sending when activating Enhanced AMR Coverage.

5 Parameters

5.1 Enhanced Handover Success Rate

5.1.1 BSC Commands

The BSC commands introduced by Enhanced Handover Success Rate are:

RFEHI Initiate Enhanced Handover Success Rate.

RFEHE End Enhanced Handover Success Rate.

RFEHP Print the status of Enhanced Handover Success Rate.

REEST Activate/deactivate Re-establish of Layer 2.

REPFACCH Activate/deactivate Repeated downlink FACCH.

For more information about the BSC commands, see Reference [2].

5.1.2 Parameters for Special Adjustment

Table 1 MSC Timers

Parameter Name Default Value

Recommended Value1

Value Range Unit

TIMHNDCMDINTRAM 15 49 5 to 120 s

HNDTCMDBASNON 15 49 5 to 120 s

THNDCMDBASICM 15 49 5 to 120 s

1The MSC Timers must be bigger than the sum T8 (BSS(BSSMAP))+T_ESTGUARD+T_REESTABLISH+T_REESTGUARD in order not to time out while Enhanced Handover Success Rate is used.

RLINKT

Actions Taken for TCH Drop (Ericsson Circle):

1. Parameter benchmarking with Indore (MP) & Mumbai (as both the circles have better sets of KPIs in Metro category).

2. Fine tuning of Power control parameters.3. Trail of different set of locating algorithm (Radio link timers).4. Trail of new features- Enhanced Handover, Enhanced AMR as suggested by E//.5. Trail of different set of timer values realated to drop due to T200 expiry as

suggested by E//.6. Implementation of SSU clock for Network Sync.

RLINKT is used by the MS to determine when to disconnect the call upon repeated failure in decoding the SACCH. RLINKT is started in the MS after the assignment of a dedicated channel. Two units increase the counter if the SACCH data is successfully decoded. One unit decreases it if it cannot be successfully decoded. If the counter reaches the value zero, then the call release is initiated.

RLINKT,RLINKTAFR,RLINKTAHR (Down link Direction)RLINKUP,RLINKUPAFR,RLINKUPAHR (Up link Direction)

RLINKT,RLINKTUP (Used for EFR,FR,HR speech codecs)RLINKTAFR, RLINKUPAFR (Used for AMR-FR speech codec)RLINKTAHR, RLINKUPAHR (Used for AMR-HR speech codec)

RLINKUP RLINKT RLINKTAFR RLINKTAHR RLINKUPAFR RLIKUPAHRType: Numeral Numeral Numeral Numeral Numeral NumeralRange: 1 to 63 8 to 64 in

steps of 48 to 64 in steps of 4

8 to 64 in steps of 4

1 to 63 1 to 63

Unit: SACCH periods (480 ms)

SACCH periods (480 ms)

SACCH periods (480 ms)

SACCH periods (480 ms)

SACCH periods (480 ms)

SACCH periods (480 ms)

Default: 16 16 16 16 16 16

Idea RAJ N/W

24 24 32 24 32 24

7. Trail TMA implementation on sample sites where drop are contributed mainly due to low signal strength UL.

8. Verification of Frequency plan (BCCH & BSIC combination).9. Change of TRX signalling from MPLEX16 to conc or unconc to remove LAPD

congestion.10. Rectification of internal alarms like VSWR, diversity etc which contributes

towards KPIs degradation.11. Augmentation of DTRU/ DRU on highly utilized cells.12. Optimization of Transcoder resources.

Observation:

1. After implementing the above mentioned point, no significant improvement is observed.

2.

3 Enhanced Handover1 Introduction1.1 Background

2 Capabilities2.1 Benefits

3 Technical Description3.1 Enhanced Handover Success Rate3.1.1 Repeated downlink FACCH3.1.1.1 Handover Command or Assignment Command not segmented3.1.1.2 Handover Command or Assignment Command segmented

3.1.2 Re-establishment of Layer 23.1.3 Value Ranges and Defaults Values

3.2 Enhanced AMR Coverage3.2.1 BSC commands3.2.2 BSC parameters

3.3 Related Statistics3.3.1 KPI impact3.3.1.1 Enhanced Handover Success Rate3.3.1.2 Enhanced AMR Coverage

3.3.2 Impact on Legacy Counters3.3.2.1 Enhanced Handover Success Rate

3.3.3 Statistics for Performance Management3.3.3.1 Enhanced Handover Success Rate3.3.3.2 Enhanced AMR Coverage

3.4 Main changes in Ericsson GSM system 07B/ BSS 07B

4 Engineering Guidelines4.1 Enhanced Handover Success Rate4.1.1 MSC timers

4.2 Enhanced AMR Coverage

5 Parameters5.1 Enhanced Handover Success Rate5.1.1 BSC Commands5.1.2 Parameters for Special Adjustment

RLINKT